Numerous devices and/or apparatus are available for detecting and/or monitoring partial electrical discharges in oil-filled devices. Such electrical discharges are detrimental since they weaken the insulation within the device resulting in the eventual failure of same. Thus, it is extremely beneficial, from both a service and an economic point of view, to detect such discharges and to monitor their frequency of occurrence.
An example of such a detection device is disclosed in U.S. Pat. No. 3,728,619 (Harold, et al) which is directed to apparatus for determining the position of corona discharges within a transformer. Typically, a corona discharge is indicative of the occurrence of a partial electrical discharge. In this reference transducer apparatus is attached to the wall of the transformer housing at a position where the ultrasonic wave created by the corona discharge may be detected. The apparatus contains two transducers which are aligned with each other in a pre-determined relationship. The apparatus is oriented so that the ultrasonic waves being detected by each transducer have a maximum amplitude and are in phase with each other. The distance between the corona source and the transducer apparatus is determined by (1) rotating the transducer apparatus a pre-determined number of degrees, (2) measuring the phase shift produced by the rotation, and (3) calculating the distance to the corona discharge using known constants and derived quantities.
The Umemura, et al reference (U.S. Pat. No. 5,075,629) discloses a partial electrical discharge detecting device for transformers which includes an antenna mounted adjacent to and exteriorly of the transformer. An electromagnetic wave resulting from the partial electrical discharge is detected by the antenna. The resulting signal produced by the antenna is supplied to a frequency spectrum analyzer to determine the existence of the electromagnetic wave. Processing circuitry is provided to determine whether or not a partial electrical discharge has occurred based upon the detected signal.
The Bruhlmeier, et al reference (U.S. Pat. No. 5,252,927) discloses a device for detecting partial electrical discharges within the windings of an electrical machine. In this case, the windings have a voltage applied thereto which is sampled over the length of the windings by means of a probe. The radio frequency signals resulting from the partial electrical discharges are received by an antenna affixed to the probe. The signals are then detected and evaluated in test equipment located in an area external to the electrical machine.
The Nishizawa, et al reference (U.S. Pat. No. 5,233,305) discloses a device for detecting partial electrical discharges which are induced the primary winding of a resin-molded transformer. In this case, the detecting sensor is connected between the transformer tap terminals and the output of the sensor is applied through amplifier circuitry to apparatus which provides an indication as to whether a partial electrical discharge has occurred.
Each of the foregoing apparatus and/or methods requires the use of either a sensor, detector or probe and a relatively complex apparatus requiring the use of rather sophisticated circuitry in order to determine whether a partial electrical discharge has occurred within the electrical device. In addition, none of the foregoing apparatus has the capability of transmitting an interrogating signal into the electrical device, and receiving a return signal from a surface or object within the device. In effect, all of the foregoing apparatus can operate only in the receive mode, and do not include any provisions for operating in the transmit mode and/or the receive and transmit modes.
In view of the foregoing, it has become desirable to develop an inexpensive ultrasonic transducer which can accurately detect and monitor partial electrical discharges in electrical devices and which can operate in the receive mode, the transmit mode and in the receive and transmit modes.